[Bug c++/104634] New: Explicit template instantiation does not work when there are multiple partial template specialization using concepts Денис Шкиря <denis.shkirja@gmail.com>

["denis.shkirja at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104634

            Bug ID: 104634
           Summary: Explicit template instantiation does not work when
                    there are multiple partial template specialization
                    using concepts  Денис Шкиря <denis.shkirja at gmail dot com>
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: denis.shkirja at gmail dot com
  Target Milestone: ---

In the code below I am trying to explicitly instantiate two different
partial specializations of a template struct with static function and
I expect to see two global weak symbols (W) that correspond to
different instantiations of that function in the object file. But for
some reason I see only one symbol that corresponds to the first
specialization. I observe this behavior on all gcc versions from 10.01 to 12 (I
checked it using godbolt - https://godbolt.org/z/4ev4ePPYe).
Also, it looks like Clang and MSVC produce both symbols.

The reproducer:
```
$ cat main.cpp
#include <concepts>

template<typename T>
struct Struct {
static void func();
};

template<std::same_as<int> T>
struct Struct<T> {
static void func() {}
};

template<std::same_as<double> T>
struct Struct<T> {
static void func() {}
};

template struct Struct<double>;
template struct Struct<int>;


$ g++ -c ./main.cpp -std=gnu++20 -o main.o
$ nm -C main.o
0000000000000000 W Struct<int>::func()
```
GCC version that I used to reproduce the problem locally:
```
$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/11/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
11.1.0-1ubuntu1~18.04.1'
--with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2
--prefix=/usr --with-gcc-major-version-only --program-suffix=-11
--program-prefix=x86_64-linux-gnu- --enable-shared
--enable-linker-build-id --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --libdir=/usr/lib
--enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-gnu-unique-object --disable-vtable-verify --enable-plugin
--enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror
--disable-cet --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-11-YRKbe7/gcc-11-11.1.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-11-YRKbe7/gcc-11-11.1.0/debian/tmp-gcn/usr
--without-cuda-driver --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.1.0 (Ubuntu 11.1.0-1ubuntu1~18.04.1)
```

I am not sure if this will be useful, but here is what the symbols
look like when compiled with clang:
```
$ /usr/lib/llvm-13/bin/clang -c ./main.cpp -std=c++20 -o main_clang.o
$ nm -C main_clang.o
0000000000000000 W Struct<double>::func()
0000000000000000 W Struct<int>::func()
```